The structure of Chromosomes of Drosophila melanogaster may be modified in a number of ways - one of the most useful and informative consists of joining whole arms together in new combinations, called compounds. Syntheses of a new type of compound chromosomes are being attempted for the first time. One of the possibilities has been successfully completed by way of a new terminal attachment placed on the left end of chromosome 2. This has made it possible to synthesize a new compound which consists of two haploid sets of chromosome 2 on a single centromere. A line carrying this chromosome behaves like a new species; its other genetic and segregational properties are being investigated.